The present invention relates to a device for measuring current in high voltage (HV) power systems.
The concept of the hybrid current transformer with various means of sensing the current and various means of making the electrical to optical conversion is known. The motivation for using optics based measurement and/or signal transmission in HV power systems is that optical signals may be transmitted by inherently insulating means such as optical fibers. The hybrid current transformer combines traditional current sensing methods, including inductive or resistive current sensing, with optical signal transmission. The known current sensing means will be discussed first, followed by optical modulation methods.
The most direct method of sensing a current is to use a shunt resistor, having low resistance, in line with the sensed current. A voltage will be generated across the resistor proportional to the current through the resistor and the resistance. This method has the advantage over the Rogowski coil (discussed below) in that it does not require time integration, and it can measure DC current in addition to AC current. An example of the shunt resistor method is shown in U.S. Pat. No. 4,629,979. When used for HV applications, this example uses active electronics in the HV environment to transmit an analog or digital optical signal from the HV to low voltage (LV) environments. In the analog case, the optical signal is generated by applying a frequency modulated carrier signal to a light emitting diode.
U.S. Pat. No. 4,070,572 uses a shunt in combination with active circuitry to amplitude modulate an LED light source to transmit the measured current signal from HV to LV. This system uses active components in the HV environment and also places the light source in the HV environment.
U.S. Pat. No. 5,446,372 (also U.S. Pat. No. 5,420,504) primarily involves the physical design of a shunt, but does include the possibility of xe2x80x9celectro-optically transmittingxe2x80x9d the measured signal using an xe2x80x9celectro-optical interface.xe2x80x9d The electro-optical interface is not described in detail but the measured signal is digitized prior to transmission.
The most common current sensing means in AC power systems is the inductive current sensor comprising a coil, which is inductively coupled with the sensed current. Within this broad class, there are two sub-classes: 1) devices which produce a secondary current which is proportional to the sensed or primary current, 2) devices which produce a voltage which is proportional to the time derivative of the sensed current. In general, the burden or load resistance placed on a coil will determine which of the two sub-classes apply. Specifically, when the inductive reactance of the coil is larger than the combined resistance of the coil and the burden, then the device will behave according to sub-class 1.
The conventional current transformer used in the power utility industry belongs to sub-class 1. These devices have no intrinsic voltage output, which makes them somewhat less appropriate for driving optical voltage sensors. A coil with sufficiently large inductance can however generate a voltage signal by placing a small resistance across the coil terminal and still behave according to sub-class 1. In this case, the voltage will be proportional to the secondary current and hence also proportional to the primary current. This method of current to voltage conversion is utilized as described by C. McGarrity, et al., AA fiber-optic system for three-phase current sensing using a hybrid sensing technique, @ Review of Scientific Instruments, Vol. 63, No. 3, pp 2035-2039, 1992, to drive an optical modulator. McGarrity""s system is passive, using a current transformer and load resistor to generate a voltage signal which is applied to an interferometer.
A disadvantage of sub-class 1 devices is that they invariably use high permeability materials in the core of the coil. In addition to making the coil heavy, high-permeability materials are generally non-linear and can saturate when measuring large fault currents.
Devices, which fit sub-class 2, are generally referred to as Rogowski coils although other names are sometimes used such as linear coupler. Occasionally, Rogowski coils are classed as current transformers although, strictly speaking, they are a time-derivative of current to voltage transformer. They will operate in this manner even with an infinite load resistance, thus producing no current at all. A load resistor is usually used and can be sized to optimize the transient response of the coil, (see D. A. Ward, J. La T. Exon, xe2x80x9cUsing Rogowski coils for transient current measurement,xe2x80x9d Engineering Science and Education Journal, June 1993, pp. 105-113) or to compensate for the thermal expansion of the core (see G. Carlson, F. Fisher, xe2x80x9cVoltage and current sensors for a 1200 kV gas insulated bus,xe2x80x9d 7th IEEE/PES Transmission and Distribution Conference and Exposition, Apr. 1-6, 1979, pp. 200-207).
In order to measure current, sub-class 2 devices must be used in combination with an integrator to recover the sensed current signal from the time-derivative. Two analogue integrator types can be used: passive or active although the passive integrator is usually only used at higher frequencies (much higher than 50 or 60 Hz power frequencies) (see D. A. Ward.) The difficulty encountered in making a passive integrator that operates at low frequencies is that as the integrator time constant is made larger, the voltage output from the integrator decreases. This can be compensated for to some extent by increasing the Rogowski coil""s output voltage (by increasing its mutual inductance) but at the expense of stressing the voltage withstand ability of the coil""s winding insulation.
For example using a single pole passive integrator for high-accuracy metering applications having a phase accuracy at 60 Hz of 5 minutes of arc, the integrator pole location should be about 1000 times lower in frequency, or at 60 mHz. The voltage signal from the Rogowski coil will be 1000 times larger than the voltage signal from the integrator i.e. to obtain a 1 V integrator signal, the Rogowski voltage will be 1000 V at 60 Hz. If a bandwidth of 6 kHz is desired, an additional factor of 100 in Rogowski coil voltage must be tolerated, pushing its voltage level to 100 kV. This number will further increase by the over-current factor that is desired. A Rogowski coil and integrator capable of such high voltage levels is not cost justifiable.
A third type of integrator can also be used by digitally sampling the time-derivative signal and subsequently digitally integrating it.
The location of the integrator is also important. If the integrator is located in the LV environment, then the time-derivative signal must be transmitted from the HV to the LV environment. This places large demands on the transmitting means in terms of dynamic range. Either active integration or digital integration can be used in this case, but both pose problems due to amplification of low frequency signal corruption introduced by the optical system.
Low frequency signal corruption can be filtered out for revenue metering applications and as such, integration in the LV environment is appropriate for metering applications.
An alternative to locating the integrator in the LV environment is to place it in the HV environment. Powering a digital or active integrator in the HV environment is not a trivial task. Several powering methods which tap power from the HV line have been used including using an auxiliary current transformer, capacitive dividers, and resistive dividers (see R. Malewski, A High-voltage current transformers with optical signal transmission,@ Optical Engineering, Vol. 20, No. 1, 1981, pp. 54-57.) All of these methods represent a finite turn-on time when energizing a line which can be a hazard when energizing a faulted line. Batteries may be used to get around this problem but with the added problem of maintaining them. Power can also be supplied independently to the integrator by optical power transmission (see D. C. Erickson, xe2x80x9cThe use of fiber optics for communications, measurement and control within high voltage substations,xe2x80x9d IEEE Transactions on Power Apparatus and Systems, Vol. PAS-99, No. 3, 1980, pp. 1057-1063.)
Non-passive integrators located in the HV environment are not only difficult to power, but can also suffer from electromagnetic interference. This further complicates their power supply design as well as the required electromagnetic shielding. A further disadvantage of non-passive integrators located in the HV environment is that they are more prone to failure due to both their use of transistor circuits as compared to only using resistor/capacitor circuits, and having considerably more components as compared to a passive integrator.
Optical Modulators
The purpose of the optical modulator is to convert the electrical signal from the current sensor into an optical signal which can readily be transmitted from the HV environment to the LV environment. Methods of achieving this objective can be divided into two broad classes. 1) devices having active components in the HV environment, 2) devices having fully passive components in the HV environment.
Examples of class 1 include U.S. Pat. Nos. 4,070,572 and 4,471,355. U.S. Pat. No. 4,070,572 uses an LED that is intensity modulated by the sensed current signal. U.S. Pat. No. 4,471,355 uses a Rogowski coil (referred to as a xe2x80x9ctoroidal coilxe2x80x9d) in combination with a transmitter located in the HV environment to send pulses encoded with amplitude and phase information representing the sensed current signal. The system disclosed in U.S. Pat. No. 4,471,355 also includes an integrator in the HV environment which is believed to be an active integrator since they have a power-supply available which is also used for the transmitter. L. Kojovic, xe2x80x9cRogowski Coils Suit Relay Protection and Measurement,xe2x80x9d IEEE Computer Applications in Power, July 1997, pp. 47-52, illustrates a current measurement system combining a Rogowski coil with analog to digital conversion. The digitized signal is transmitted optically from the HV to LV environments.
Several passive optical modulation methods have been developed for hybrid current measurement.
U.S. Pat. No. 4,894,609 (expired) and 5,012,182 describe a porcelain insulator with an electro-optic modulator combined with a current transformer. The current transformer generates a voltage signal by passing its secondary current through a load resistance. The voltage signal is then applied to a bulk Pockels cell. Two disadvantages of this sensor are 1) it has low sensitivity due to the bulk crystal used and the associated large electrode separation, 2) the sensor provides only a single optical signal and therefore the optical signal cannot be normalized in the manner that a dual channel Pockels cell can. Although dual channel bulk Pockels cells are well known, this increases the complexity of the optical components located in the HV environment. This is disadvantageous because components in the HV environment must endure harsh environmental conditions and as such are more prone to failure and also the temperature stability of the optical components can affect measurement accuracy.
U.S. Pat. No. 4,376,247 describes a remote current sensor using a liquid crystal attenuator to modulate light intensity. The current signal is obtained by use of a current transformer. U.S. Pat. No. 3,662,263 describes a method of obtaining optical modulation for the purpose of encoding a sensed current signal by phase modulating one of two coherent optical paths and subsequently combining the paths to obtain an optical intensity modulation.
The McGarrity reference referred to above describes a method of obtaining optical modulation for the purpose of encoding a sensed current signal using a fiber optic Michelson interferometer. A PZT stretcher modulates one of the path lengths in the interferometer.
U.S. Pat. No. 5,103,164 issued Apr. 7, 1992 to Kawaguchi et al. describes one such method in which a Faraday sensor is enclosed in a solenoid, and the solenoid is connected to a Rogowski coil. The solenoid produces a magnetic field proportional to the sensed current. The Faraday sensor then senses this magnetic field and produces an optical modulation. U.S. Pat. No. 5,103,164 also discusses the application of the Rogowski coil signal to a load resistor and a Pockels element. The type of Pockels element is not specified explicitly but the signal levels given in the preferred embodiment are representative of a bulk-optic Pockels cell and are considerably larger than the signal levels preferred for the present invention. Kawaguchi""s invention, with the Pockels element embodiment, does not provide a means to integrate the time derivative signal generated by the Rogowski coil.
An overview of optical current sensors can be found in: Emerging Technologies Working Group, Fiber Optic Sensors Working Group, xe2x80x9cOptical Current Transducers for Power Systems: A Review,xe2x80x9d IEEE Transactions on Power Delivery, Vol. 9, No. 4, 1994, pp. 1778-1788.
U.S. Pat. No. 5,029,273 describes the integrated-optic Pockels cell (IOPC) and is discussed hereinbelow.
It is an object of the present invention to provide a hybrid optical current sensor for measuring current in a high voltage current carrier.
It is also an object of the present invention is to provide a system capable of measuring current in HV power systems with sufficient accuracy and dynamic range for power metering.
It is also an object of the present invention to provide system with a frequency response and dynamic range sufficient for protective relaying.
Broadly the present invention relates to a method and/or apparatus for measuring current in a HV current carrier comprising a current to voltage transducer for generating a low voltage signal representative of said current in said HV carrier, applying said low voltage signal to an integrated-optic voltage sensor, said current to voltage transducer and said integrated-optic voltage sensor located in a HV environment adjacent to said HV current carrier to produce a modulated optical signal representative of said current being measured and conducting said optical signal to a LV environment insulated from said HV environment, processing said optical signal to provide a second electrical signal representative of said current.
In a preferred embodiment, said current to voltage transducer is a shunt resistor, and said low voltage signal and said second electrical signal are substantially proportional to said current.
In another preferred embodiment, said current to voltage transducer is a current transformer with a load resistor, and said low voltage signal and said second electrical signal are substantially proportional to said current.
In another preferred embodiment, said current to voltage transducer is a Rogowski coil, and both said low voltage signal and said second electrical signal are substantially proportional to the time derivative of said current.
In another preferred embodiment, said current to voltage transducer is a Rogowski coil, said low voltage signal is substantially proportional to the time derivative of said current, and said second electrical signal is integrated in the LV environment to provide a third electrical signal substantially proportional to said current.
In another preferred embodiment, said current to voltage transducer is a Rogowski coil, said low voltage signal, which is substantially proportional to the time derivative of said current, is passively integrated to provide a modulating signal that is applied to said integrated-optic voltage sensor, and said modulating signal and said second electrical signal are substantially proportional to said current.
In another preferred embodiment, said current to voltage transducer is a Rogowski coil, said low voltage signal is passively integrated by a first integrator over at least one portion of said apparatus"" bandwidth to provide a modulating signal which is representative of said current, and said second electrical signal is integrated in the LV environment by a second integrator over those at least one portions of said bandwidth that have not been integrated by said first integrator, to provide said third electrical signal which is substantially proportional to said current.
Preferably all components located in the HV environment are passive.
Preferably said current to voltage transducer consumes little power, is light in weight, and does not generate substantial heat.
Preferably said low voltage signal is conditioned before application to said integrated-optic voltage sensor.
Preferably said integrated-optic voltage sensor""s bias is used to compensate for thermal variation of components located in the HV environment.
Preferably said integrated-optic voltage sensor is an IOPC.
Preferably said IOPC has at least two electrodes.
Preferably said at least two electrodes are located on the surface of an IOPC substrate on which a waveguide is formed.
Preferably two electrodes are located on said surface, said waveguide is located between said two electrodes, said substrate is X-cut lithium niobate, and said waveguide is directed along the crystallographic Z direction.